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.Dd $Mdocdate: November 2 2019 $
.Dt CMS_ADD1_SIGNER 3
.Os
.Sh NAME
.Nm CMS_add1_signer ,
.Nm CMS_SignerInfo_sign
.Nd add a signer to a CMS SignedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft CMS_SignerInfo *
.Fo CMS_add1_signer
.Fa "CMS_ContentInfo *cms"
.Fa "X509 *signcert"
.Fa "EVP_PKEY *pkey"
.Fa "const EVP_MD *md"
.Fa "unsigned int flags"
.Fc
.Ft int
.Fo CMS_SignerInfo_sign
.Fa "CMS_SignerInfo *si"
.Fc
.Sh DESCRIPTION
.Fn CMS_add1_signer
adds a signer with certificate
.Fa signcert
and private key
.Fa pkey
using message digest
.Fa md
to the
.Fa signerInfos
field of the
.Vt SignedData
structure
.Fa cms ,
which should have been obtained from an initial call to
.Xr CMS_sign 3
with the flag
.Dv CMS_PARTIAL
set, or which can be a valid
.Vt SignedData
structure in the case of re-signing.
.Pp
If
.Fa md
is
.Dv NULL ,
the default digest for the public key algorithm of
.Fa pkey
is used.
.Pp
Unless the
.Dv CMS_REUSE_DIGEST
flag is set, the
.Fa cms
structure remains incomplete and must be finalized either by streaming
(if applicable) or by a call to
.Xr CMS_final 3 .
.Pp
The main purpose of
.Fn CMS_add1_signer
is to provide finer control over a CMS
.Vt SignedData
structure where the simpler
.Xr CMS_sign 3
function defaults are not appropriate, for example if multiple signers
or non default digest algorithms are needed.
New attributes can also be added using the returned
.Vt CMS_SignerInfo
structure and the CMS attribute utility functions or the CMS signed
receipt request functions.
.Pp
Any of the following flags (OR'ed together) can be passed in the
.Fa flags
parameter:
.Bl -tag -width Ds
.It Dv CMS_REUSE_DIGEST
Attempt to copy the content digest value from one of the existing
.Vt CMS_SignerInfo
structures in
.Fa cms
while adding another signer.
An error occurs if a matching digest value cannot be found to copy.
The
.Fa cms
structure will be valid and finalized when this flag is set.
.It Dv CMS_PARTIAL
If this flag is set in addition to
.Dv CMS_REUSE_DIGEST ,
the returned
.Vt CMS_SignerInfo
structure will not be finalized so additional attributes can be added.
In this case an explicit call to
.Fn CMS_SignerInfo_sign
is needed to finalize it.
.It Dv CMS_NOCERTS
Do not add the signer's certificate to the
.Fa certificates
field of
.Fa cms .
The signer's certificate must still be supplied in the
.Fa signcert
parameter though.
This flag can reduce the size of the signature if the signer's certificate can
be obtained by other means, for example from a previously signed message.
.It Dv CMS_NOATTR
Leave the
.Fa signedAttrs
field of the returned
.Vt CMS_SignedData
structure empty.
By default, several CMS
.Vt SignedAttributes
are added, including the signing time, the CMS content type,
and the supported list of ciphers in an
.Vt SMIMECapabilities
attribute.
.It Dv CMS_NOSMIMECAP
Omit just the
.Vt SMIMECapabilities
attribute.
.It Dv CMS_USE_KEYID
Use the subject key identifier value to identify signing certificates.
An error occurs if the signing certificate does not have a subject key
identifier extension.
By default, issuer name and serial number are used instead.
.El
.Pp
If present, the
.Vt SMIMECapabilities
attribute indicates support for the
following algorithms in preference order: 256 bit AES, Gost R3411-94,
Gost 28147-89, 192 bit AES, 128 bit AES, triple DES, 128 bit RC2, 64 bit
RC2, DES and 40 bit RC2.
If any of these algorithms is not available then it will not be
included.
.Pp
The
.Fn CMS_SignerInfo_sign
function explicitly signs
.Fa si .
Its main use is when the
.Dv CMS_REUSE_DIGEST
and
.Dv CMS_PARTIAL
flags were both set in the call to
.Fn CMS_add1_signer
that created
.Fa si .
.Sh RETURN VALUES
.Fn CMS_add1_signer
returns an internal pointer to the new
.Vt CMS_SignerInfo
structure just added or
.Dv NULL
if an error occurs.
.Sh SEE ALSO
.Xr CMS_ContentInfo_new 3 ,
.Xr CMS_final 3 ,
.Xr CMS_sign 3 ,
.Xr ERR_get_error 3
.Sh STANDARDS
RFC 5652: Cryptographic Message Syntax, section 5.1: SignedData Type
.Pp
RFC 8551: Secure/Multipurpose Internet Mail Extensions (S/MIME)
Version\ 4.0 Message Specification
.Bl -dash -compact -offset indent
.It
section 2.5: Attributes and the SignerInfo Type
.It
section 2.5.2: SMIMECapabilities Attribute
.El
.Sh HISTORY
.Fn CMS_add1_signer
and
.Fn CMS_SignerInfo_sign
first appeared in OpenSSL 0.9.8h
and have been available since
.Ox 6.7 .
